The present invention relates to a method and apparatus for dispersing a volatile composition into the air and, in particular, to a method and apparatus which relies upon an ion wind to facilitate the dispersal into the air of one or more volatile compounds from a source of a volatile composition.
Compositions which are frequently dispersed into the air include insect repellents, insecticides and air freshening or room fragrancing compositions.
Chemical insect repellents are known in the art and are widely used. For example, N,N-diethyl-m-toluamide (DEET) is widely used as an insect repellent for use on clothing and the skin to repel insects which bite, such as mosquitoes. Citronella oil and eucalyptus oil are also used for the same purpose. However, the application of such chemicals has disadvantages in that they need to be frequently reapplied and they can produce allergic responses in some people.
Pesticides, such as synthetic pyrethroids also have a repellent and/or insecticidal action and can be used to treat clothing, mosquito nets etc. However, prolonged or frequent exposure to synthetic insecticides may be hazardous to health.
Alternatively, insects can be excluded from contact with human beings by providing physical barriers, such as netting or fly screens, over windows and doors, or mosquito netting around beds. The disadvantage of such physical barriers is that the entry of air is severely restricted when the barriers are in place because of the small mesh size required to exclude the insects. This leads to discomfort in hot climates.
Another alternative for use in enclosed spaces, particularly for use overnight, is to burn an insect coil for example containing an insecticidal composition containing a pyrethroid active agent which may also have a repellent effect. Alternatively, an electrical device may be used in which insecticidal tablets containing an insecticidal composition such as a pyrethroid active agent which may also have a repellent effect are heated electrically so that the insecticide/repellent evaporates into the air space and repels and/or kills insects, in particular mosquitoes.
Ultrasound devices have also been sold for repelling mosquitoes, but their efficacy has not been scientifically proven.
Various methods are known for the dispersion of fragrance compositions, such as air fresheners, into a space. For example, an aerosol device may be used to dispense an aerosol spray of the fragrance composition. A disadvantage of such devices is that the fragrance generally only has an effect within the direction of the line of spray and does not last for very long. Other methods of delivering fragrance composition into a space include:                (a) natural evaporation of a liquid fragrance composition delivered to, and exposed to, the atmosphere by means of a porous wick;        (b) natural evaporation and decomposition of a solid gel which includes the fragrance composition; and        (c) enhanced evaporation of a liquid fragrance composition by local heating of a wick delivery system.        
In general, these methods simply distribute a fragrance within an enclosed environment, the sole purpose being to create a perfumed atmosphere.
Ion winds are known in the art and an ion wind is generated as a direct result of the interaction between negatively or positively charged ions and air molecules. Ion winds are described and explained in “Electrostatics: Principles, Problems and Applications”, J. A. Cross, 1987, Adam Hilger, pp 278-284.
Ion winds may be generated using an electrode arrangement in which a first electrode has one or more sharp points and a second electrode acts as an opposing electrode. If the electric field at the tip of the sharp point or points of the first electrode exceeds the breakdown field of air (approximately 30 kV/cm) then electrical breakdown of the air will occur for either an ac or dc potential applied to the electrode. This phenomenon is generally termed “corona discharge”.
For a dc potential, ions which are of opposite polarity to that of the first electrode will be attracted to the first electrode and collected. Ions of the same polarity to that of the first electrode will be repelled by it, and will be attracted towards the second electrode. The ions are of approximately the same size as neutral air molecules and since the ions which are attracted to the second electrode are under the influence of an electrical field (E), a force of F=qE will be exerted on them which causes the air molecules to move. As the ions move, they collide with neutral air molecules and momentum sharing occurs. This in turn imparts a force on the neutral air molecules thus inducing movement in the same direction. This is known as the “ion-drag” effect and is the mechanism which leads to the bulk movement of air, otherwise termed an “ion wind”. Unidirectional airflow will be induced in this way both for +ve and −ve dc potentials.
In an alternating field (ac) ionisation will still occur but there will be no net movement of ions in one direction and thus no ion wind generation.
GB-A-2066076 describes an apparatus in which both positive and negative ionic species are generated using a plasma which is generated using radio-frequency methods.
WO92/15339 describes an apparatus in which an electrostatic charge is applied to a wick system. This results in the formation of a “Taylor” cone at the extremity of the fibres of the wick which causes' atomisation of the liquid from the wick.
SU-A-1803679 describes the use of an electrically driven fan to blow ionized air over a pine tree in order to disperse vapours from the tree into the air.
None of the prior art devices results in a unidirectional induced airflow arising from momentum transfer and hence there is no ion wind produced in the prior art for product dispersion.
We have now developed a method and apparatus using an ion wind whereby a volatile composition may be more effectively distributed throughout a particular space.